1. Field of the Invention
The present invention relates to a 2-cycle internal combustion engine and, more particularly, to an improvement in the configuration of an exhaust port of a 2-cycle internal combustion engine.
2. Description of the Related Art
FIG. 8 is a vertical sectional view of a 2-stroke motorcycle engine having a cylinder which is inclined forwardly and which is equipped with an exhaust port of a conventional configuration. As shown in this figure, an exhaust port 11 is formed so as to open in the front portion of the inner peripheral surface of the cylinder 1 which is inclined forwardly. The exhaust port is connected, through an exhaust passage 12 formed in the cylinder wall, to an exhaust pipe (not shown) which is connected to a front part of the cylinder. The engine has a crankshaft 9 which rotates in the direction of arrow R.
As shown in FIG. 9 or 10, the exhaust port 11 has an elliptic or rectangular form.
Referring again to FIG. 8, the engine has a piston 5 which is shown in its upward stroke. In this state, a connecting rod 8 is inclined such that its big end is positioned at the rear side of an axis C1 of the cylinder 1. Consequently, the piston 5 also is inclined with respect to the cylinder axis C1 in the same direction as the direction of inclination of the connecting rod 8. Therefore, the front side of the piston 5 makes contact with the inner peripheral surface 3 of the cylinder at a point or portion P1 which is intermediate between the top and bottom ends of the piston 5, while the rear side of the piston 5 contacts with the cylinder inner peripheral surface 3 at the bottom end portion P2.
In operation, the portion P1 of the piston 5 falls into the exhaust port 11 immediately after it passes the lower edge of the exhaust port 11 during an upward stroke of the piston 5. The portion P1 then collides with the upper edge of the exhaust port 11 as a result of further upward movement of the piston 5. Consequently, an annoying rattling noise is generated during the operation of the engine.
FIG. 11 is a chart showing the levels of vibration and noise as measured at a position S shown in FIG. 8. The upper part of the chart shows the timing or piston position in terms of pulses generated in response to detection of TDC (Top Dead Center), while the lower part of FIG. 11 shows the level of the measured noise. The middle part of FIG. 11 shows the level of vibration.
The falling of the portion P1 of the piston 5 into the exhaust port 11 takes place when the piston is at .theta..degree. before TDC so that a vibration takes place as indicated at X in the middle part of FIG. 11. A striking noise is then generated with a slight delay after the vibration, as indicated by Y in the lower part of FIG. 11.
The impact generated as a result of falling of the piston into the exhaust port and consequent vibration and noise are large particularly in a 2-cycle motorcycle engine which has a forwardly inclined cylinder.
Japanese Utility Model Unexamined Publication No. 54-156019 discloses an exhaust port configuration in which a vertical elongated rib, having a scavenging port, is provided at the center of the suction port. The piston, however, never falls into the scavenging port due to the specific relation between the port position and the direction of crankshaft rotation. Thus, the rib merely serves to provide a scavenging port, and is not intended to provide means preventing the piston from falling into an exhaust port.